1. Field of the Invention
The invention relates to a lock-up clutch hydraulic pressure supply apparatus, and in particular, it relates to a lock-up clutch hydraulic pressure supply apparatus that reduces the current used to drive an electromagnetic valve.
2. Description of the Related Art
In vehicles, a torque converter is provided between an engine and an automatic transmission. In order to improve the torque transfer efficiency, some torque converters are provided with a lock-up clutch. A lock-up clutch is typically controlled using the hydraulic pressure output from an electromagnetic valve. In response to the hydraulic pressure being output from the electromagnetic valve, the lock-up clutch is switched from one of the released state and the applied state to the other.
When the electromagnetic valve continues to be energized, the electrical power consumption is large and it therefore reduces the fuel economy. To counter this, technologies have been proposed which improve the fuel economy by reducing the current used to drive the electromagnetic valve.
Japanese Patent Application Publication (No. 07-127730) describes a solenoid control apparatus for a vehicle lock-up clutch. In order to improve the fuel economy, this solenoid control apparatus maintains the lock-up clutch in the applied state by driving a linear solenoid (electromagnetic valve) with a reduced current. More specifically, when establishing each control mode of the lock-up clutch, that is, the applied mode, the released mode, the slip mode, and the fast-released mode, the solenoid control apparatus drives a particular solenoid with a drive current Isol. The drive current Isol is set within a range that satisfies IsolA<Isol<IsolB in the slip mode, IsolA>Isol in the applied mode, and IsolB<Isol in the fast-released mode. The solenoid control apparatus has a determination portion and a setting portion. The determination portion determines based on the operation state of the vehicle whether the vehicle is likely to be stopped. When it is determined that the vehicle is likely to be stopped, the setting portion sets the drive current Isol to a level close to the lower limit value IsolA of the slip mode, and when it is determined that the vehicle is not likely to be stopped, the setting portion sets the drive current Isol to zero or to a level near zero.
According to this solenoid control apparatus, as described above, when it is determined that the vehicle is not likely to be stopped, the drive current is set to the level near zero, and therefore the power consumption decreases accordingly. As such, the electrical power consumption is reduced.
However, the above-described reduction of the drive current to the electromagnetic valve can be accomplished only when the current to the electromagnetic valve is lower than IsolA, that is, when the electromagnetic valve is not required to output hydraulic pressure. Therefore, when the current to the electromagnetic valve can not be reduced to the level close to zero, that is, when the electromagnetic valve is required to output hydraulic pressure, the above-described reduction of the drive current to the electromagnetic valve can not be accomplished.